The present invention relates generally to signal processing, and more particularly to a temperature-compensated peak detector for low amplitude approximately periodic signals.
A common circuit found in signal processing applications is a peak detector. The function of a peak detector is to receive an AC input signal and output a DC signal that is proportional to the peak voltage of the AC input signal.
The basic peak detector employs a diode, a resistor and a capacitor to rectify the AC component of the input signal. The capacitor is charged to the peak of the input signal and slowly discharges during the negative slope of the AC input signal, with a time constant determined by the values of the capacitor and resistor. With a time constant much larger than the period of the AC input signal, the peak detector produces a relatively smooth DC output signal with a small ripple voltage.
The basic peak detector circuit is inapplicable to small voltage applications as the input signal must be sufficiently large to overcome the threshold (or forward bias) voltage of the diode.
Accordingly, it has been necessary to design peak detector circuits that are biased by an external source. The input signal is then added to the external bias (or offset) voltage and applied to the diode (or other semiconductor junction). Such a circuit then requires that the influence of the external bias voltage be removed or cancelled from the DC output so as to arrive at a signal proportional to the peak of the AC input signal.
An important parameter in determining a system""s bit-error-rate performance is the system""s extinction ratio. This parameter is the ratio between the signal power level during a logical binary xe2x80x9chighxe2x80x9d and the signal power during a logical binary xe2x80x9clowxe2x80x9d. It is essentially a measurement of the extent to which a logical 1 is distinguishable from a logical 0. The extinction ratio is usually expressed in decibels.
For commercial applications, it is desirable that a circuit have an extinction ratio higher than 8.2 dB for all operating temperatures. For operating temperature design parameters of xe2x88x9210 C. to 70 C., it is desirable that the circuit have an extinction ratio of about 10 to 11 dB under normal conditions, with a variation of 1 to 2 dB due to temperature changes.
It has been found that the semiconductor components commonly used in peak detector designs exhibit a non-linear temperature variation that can cause unacceptably large variations in the extinction ratio. In order to address the temperature problem, a non-linear compensation solution is necessary. It is advantageous to solve the temperature compensation problem with a simple design to minimize the number of components necessary and to preserve board space and reduce the overall circuit cost.
The present invention provides a system and method of obtaining a signal proportional to the peak value of an approximately periodic low amplitude input signal that compensates for non-linear temperature variations.
In one aspect, the present invention provides a peak detector that receives an approximately periodic input signal and produces an output signal proportional to the peak of the input signal, the peak detector includes an exponential operator for receiving the input signal and generating a modulated signal proportional to an exponential function of the input signal, an averaging circuit for receiving the modulated signal and generating an average signal proportional to the average amplitude of the modulated signal and a temperature compensator for adjusting the average signal to eliminate temperature-dependent characteristics from the average signal and thereby produce the output signal.
In another aspect, the present invention provides a method of obtaining an output signal proportional to the peak value of an approximately periodic input signal, comprising the steps of receiving the input signal at an input port, generating a modulated signal proportional to an exponential function of the input signal, generating an average signal proportional to the average value of the modulated signal, compensating for temperature-dependent characteristics contained in the average signal and producing the output signal at an output port.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.